Implantable defibrillators, cardioverters, and antitachycardia pacemakers, or devices which combine two or more of these functions, require tachyarrhythmia detectors. The objective of the detector is to detect and signal the occurrence of a cardiac depolarization in the tissue adjacent to one or more electrodes in contact with or in proximity to the heart. Direct cardiac leads are those in such contact with heart tissue, as contrasted to electrodes that are non-invasive, i.e., secured to the skin, to monitor electrical cardiac activities.
It is essential to blank (isolate) the sensing amplifier of detection to protect against saturation due to pacing pulses, and to provide other types of protection against damage or malfunction due to external interference sources. These techniques are familiar to those skilled in the art, and are outside the scope of the below-described invention. Similarly, it is important to provide symmetric positive and negative comparator thresholds, or full wave rectification and a single comparator, for detecting both signal polarities. These techniques also are familiar to those skilled in the art, and are outside the scope of the below-described invention.
Langer U.S. Pat. No. 4,184,493 refers to a sensing circuit for an implantable defibrillator, having an amplifier, a first-order high-pass filter (the corner frequency is undisclosed), a bidirectional comparator, and a circuit which measures the fraction of time that the comparator asserts its output. The detector measures the average fraction of time when the ECG exhibits a high slope, compared to the fraction of time when the ECG exhibits a low slope. The circuit provides feedback to adjust amplifier gain automatically to maintain constant amplitude at filter output.
Imran U.S. Pat. No. 4,393,877 refers to a sensing circuit for an implantable defibrillator, which includes a slew rate detector circuit and a zero-crossing detector circuit. The slew rate detector has an amplifier, a first-order high-pass filter (corner frequency undisclosed), a unidirectional comparator, and a monostable multivibrator for timing a refractory period. The Imran circuit provides feedback to adjust amplifier gain automatically, to maintain constant amplitude at the filter input.
Mower U.S. Pat. No. 4,559,946 refers to a sensing circuit for an implantable defibrillator, which includes a slew rate sensitive circuit followed by a comparator. The Mower circuit provides feedback to adjust automatically, the comparator threshold to a fraction of recent peak filter output
Menken U.S. Pat. No. 4,819,643 refers to a defibrillator with stimulation (pacing) and detection (sensing) functions, and a separate channel for the detection of fibrillation with automatic gain control, which delays the stimulation until the gain of the fibrillation detection channel reaches its maximum sensitivity. Menken does not provide any details of the bandwidth of the amplifier.
Grevis U.S. Pat. No. 4,940,054 refers to a defibrillator having an amplifier with at least two values of sensitivity, which are selected as a function of the type of rhythm detected. Grevis does not provide any details of the bandwidth of the amplifier.
Baker U.S. Pat. No. 4,880,004 refers to a circuit for arrhythmia detection, which includes an input amplifier having a bandwidth that is not specified, followed by a sensing channel with a high-pass filter at 25 Hz, and a measuring channel with additional high-pass filtering at 12 Hz. Each channel is followed by comparators. The Baker circuit provides feedback to adjust amplifier gain automatically to maintain constant the sensing margin (the amount by which the signal in the sensing channel exceeds its threshold). Due to the additional filtering in the measuring channel, this margin improves at low frequencies.
Carroll U.S. Pat. No. 4,967,747 refers to an implantable defibrillator with a sensing circuit a switched capacitor gain/filter block with an unspecified bandwidth, designed to provide glitch-free gain changes for automatic gain control.
Keimel U.S. Pat. No. 5,117,824 refers to an implantable circuit for stimulation and detection with automatic gain control, but which does not adjust the threshold after the stimulation. Keimel does not provide any details of the bandwidth of the amplifier.
In "State-of-the-Art of the AICD", Pace, May 1991, Winkle refers to a defibrillator without automatic gain control, for which "programming sensitivity requires great care at time of implantation and during follow-up to be certain that T waves are not oversensed and most importantly that ventricular fibrillation/tachycardia is not undersensed."
In "Failure of a Second and Third Generation Implantable Cardioverter to Sense Ventricular Tachycardia: Implications for Fixed-Gain Sensing Devices", Pace, May 1992, Sperry et al. wrote that "more sensitive fixed-gain settings or automatic-gain sensing are needed to detect low amplitude signals on a consistent basis. The undesirable aspect of using such high sensitivity is that of oversensing (e.g. T waves) by the device resulting in an increased risk of inappropriate discharges. Failure to sense ventricular fibrillation has however also been reported with automatic gain devices as well."
The inventors have recognized that a sensing circuit for a tachyarrhythmia detector needs to detect cardiac depolarizations, but reject undesired signals, including cardiac repolarization (T wave), and including signals emanating from the return to equilibrium of the electrode system and the pacemaker's output circuit after pacing, called pacing artifact. The aforementioned prior art techniques consist of high pass filters, refractory periods, and automatic gain or threshold control; but shortcomings exist for each of these, as follows.
Shortcomings of high pass filters: A filter for an implantable life-support device should have minimum complexity necessary to meet two performance criteria: Acceptably low probabilities of (i) rejecting desired signals and (ii) detecting undesired signals. None of the prior art patents discloses any systematic method for selecting filter characteristics (parameters) such as order, quality factor Q, and corner frequencies, to meet preselected performance criteria, with minimally complex hardware.
Shortcomings of refractory periods: If designers select a refractory period length sufficiently short to permit detection of tachyarrhythmias, then it turns out the refractory period does not cover T-waves at low heart rates (the time from depolarization to repolarization increases with decreasing heart rate). If refractory periods alone sufficed, the prior art devices would not provide additional complex automatic gain or threshold control.
Shortcomings of automatic gain or threshold control: Detectors can improve selectivity (the probability of rejecting undesired signals) and sensitivity (the probability of detecting desired signals) with this technique, only when a detected characteristics of recent events are good predictors of the same characteristics in future events.
There thus remains a need for improvement in the detection (sensing) response of implantable defibrillators.